The haplodiploidy threshold and social evolution

Abstract

Workers in eusocial insect species help in rearing their siblings and other genetic relatlves instead of producing their own offspring. The multiple origins of such eusociality in the Hymenoptera have been ascribed to aplodiploidy because this genetic system makes a female more closely related to her full sisters than she would be to her offspring. To test this so called haplodiploidy hypothesis, I first assume that workers are capable of investing in their sisters and brothers in the ratio that is optimal for them. I then define a haplodiploidy threshold as that value of genetic relatedness between workers and their sisters such that they have a weighted average relatedness to the brood they rear of 0.5 and thus have the same fitness as solitary nesting females. Using 177 published estimates of relatedness between sisters and in social hymenopteran, colonies, I show that in 29 out of 35 species studied, there is not even one estimate of relatedness that is significantly higher than the haplodiploidy threshold. I conclude therefore that the multiple origins of eusociality in the Hymenoptera cannot be ascribed solely to the genetic asymmetry created by haplodiploidy.